1. Field of the Invention
The present invention generally relates to an image forming apparatus having a laser array light source.
2. Description of the Related Art
Conventionally, electrophotographic image forming apparatuses have employed a scanning optical method to form a scanning line on a photosensitive body by irradiating a rotating polygon mirror with a laser beam to scan onto the photosensitive body. However, the number of rotations, response speeds of beam pulses and the photosensitive body, and the like of this scanning optical method are not becoming high enough to meet market demands for electrophotography at higher speed.
Therefore, an edge emitting type array laser head having plural light emitting units provided linearly as disclosed in Patent Document 1 and a surface emitting type laser head having plural light emitting units arranged in a matrix as disclosed in Patent Document 2 are being developed as a laser device capable of emitting more laser beams at the same time.
When channels of laser beams are increased in such laser devices, distortions of an optical system such as skewed and bowed scanning lines, are generated. To solve these problems, there has been suggested a method to digitally correct a laser irradiation address by taking advantage of the fact that digital laser irradiation addresses can be formed more precisely (with smaller pitch) as disclosed in Patent Document 3.
The aforementioned method related to the correction of skew has been suggested not only for a scanning optical type electrophotographic apparatus, but also for an image forming apparatus which writes data of one raster all at once by using a long LED as disclosed in Patent Document 4. For the correction of skew, there has been also disclosed a method to moderate generation of a step in a line in a main scanning direction caused by the correction by distributing exposure strength between adjacent rasters, besides the aforementioned method.
Patent Document 5 discloses a configuration to repeat discontinuous exposure of each exposure area by one scan, by driving a semiconductor laser so that an exposed area and a non-exposed area periodically appear in each main scan, with a recording density in a main scanning direction set at 400 dpi and a recording density in a vertical direction set at 3200 dpi which is twice as high as that of a first embodiment thereof, in order to reduce a heating value by reducing a duty ratio, to suppress an effect by droop.
Patent Document 6 discloses a control method to shift an image in stages in a vertical scanning direction as a moving direction of a latent image support and shift an image at different positions in plural scanning for one scanning line in multiple exposure, or a control method to draw an image of image data on a printing unit by dividing the image data into plural parts with respect to a main scanning direction that is vertical to a moving direction of the latent image support so that a boundary between the parts is not linear in the vertical direction, and shifting the parts in the vertical scanning direction in stages.    Patent Document 1: Japanese Patent Application Publication No. 2001-264657    Patent Document 2: Japanese Patent Application Publication No. 2004-276532    Patent Document 3: Japanese Patent Application Publication No. 2007-168236    Patent Document 4: Japanese Patent Application Publication No. 2006-142787    Patent Document 5: Japanese Patent Application Publication No. 7-32647    Patent Document 6: Japanese Patent Application Publication No. 2006-255958
In the case of increasing the number of beam channels by using, for example, an array type laser head in Patent Documents 1 to 4, there is a problem in that changes in the amount of light (droop) are increased due to increased temperature of such light sources.
Moreover, when the number of beam channels is increased, the number of scanning lines scanned at the same time is increased. Therefore, the pitch of scanning becomes larger. As a result, there is a problem in that skew is increased. The problem of skew can be solved by digitally correcting laser irradiation addresses as in the conventional technique. To solve the problem of droop as well, however, a process to turn off each channel of the plural beam channels is also required in combination.
In the method disclosed in Patent Document 5, there are following problems. The method of Patent Document 5 cannot solve both correction of skew of a scanning line generated by multi-beam scanning, and elimination of droop. In particular, Patent Document 5 discloses a configuration in which two main scans corresponds to one scanning line. In this case, the duty ratio is limited to 50% at highest. There is no description in Patent Document 5 as to realizing a higher duty ratio without degrading quality of the scanning line.
Further, there is no description as to reducing droop in Patent Document 6, in which a method to reduce a defect involved with correction of skew is described.